Dual-mode AFC circuit for an SSB radio transceiver

ABSTRACT

An automatic frequency control (AFC) circuit is disclosed for use in an SSB radio transceiver having the capability of transmitting and receiving SSB and FM data signals over any one of a number of communication channels having equal bandwidths. The dual-mode AFC circuit includes AFC circuitry which is capable of tracking the frequency of either: (a) a first radio frequency (RF) carrier having voice signals modulated via a signal sideband amplitude modulation within a first channel bandwidth by locking to the reduced pilot carrier transmitter with the SSB signals; or of (b) a second RF carrier having high speed data signals modulated by narrowband frequency modulation within the designated channel bandwidth. The AFC circuit also includes control circuitry to memorize the AFC control voltage during the time the mobile is transmitting, and to assist the acquisition of the FM data carrier by varying the AFC sweep rate on the signalling channel.

CROSS-REFERENCE TO RELATED APPLICATION

This application contains subject matter related to co-pendingapplications Ser. Nos. 06/926,022 and 06/926,285 both filed Oct. 31,1986, and both assigned to the same Assignee as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to single sideband radiocommunications systems. More specifically, the present invention relatesto technique for transmitting analog voice signals via single sidebandamplitude modulation, and digital data signals via narrowband frequencymodulation, in a single trunked radio system having dedicated channelbandwidths.

2. Description of the Prior Art

In recent years, there has been a renewed interest in the research anddevelopment of a variety of narrowband communication systems. Thiseffort has been stimulated by the severe spectrum congestion beingexperienced by the land mobile radio services in major metropolitanareas with the present amplitude modulation (AM) and frequencymodulation (FM) systems. In the United States, 25 or 30 kHz FM systemsare used throughout the VHF (very high frequency) and UHF (ultrahighfrequency) bands, while in other countries, including Great Britain,channel spacings of 12.5 kHz are also used. A further reduction inchannel spacing to less than 12.5 kHz is considered necessary if futuredemands for spectrum are to be satisfied.

Digital voice transmission which employs linear predictive coding (LPC)and single sideband (SSB) have emerged as potential solutions to theproblem of increasing spectrum congestion. For example, Carney andLinder, in their article entitled, "A Digital Mobile Radio for 5-6 kHzChannels", IEEE International Conference on Communications,Philadelphia, Pa., June 13-17, 1982, describe a 2400 bits-per-second(bps) LPC voice encoding technique for VHF-FM land mobile radios. Bitrate reduction is realized with LPC by removing redundancy throughcomplex speech analysis and synthesis. Decreasing the high frequencycontent of the data, while lengthening the duration of the bittransitions, reduces the baseband data bandwidth such that a channelspacing of 5 to 6.25 kHz is feasible. However, there is a noticeabledegradation in speech quality attributable to the speech encodingtechnique.

It has been known for many years that single sideband modulation has theadvantage of a reduction in occupied bandwidth over FM systems orstandard AM systems. SSB is, in reality, amplitude modulation with acarrier and one of the two sidebands suppressed, utilizing only onesideband to convey information from the transmitter to the receiver. Thereceiver, to demodulate the SSB signal, must recreate the suppressedcarrier with the same frequency relationship to the single sideband asthat of the original carrier in order to prevent distortion of thedemodulated information. Voice information will sound severely distortedif the frequency error is much greater than ±20 Hz. Furthermore, avariation in RF signal strength due to propagation conditions results ina corresponding variation in the detected audio level of the SSB signl.This causes a severe degradation of voice intelligibility.

If, however, a pilot signal is transmitted continuously with the singlesideband message, and used by the receiver to track and eliminate anyfrequency variation imposed by fading, no frequency shift will occur inthe demodulated signal. Furthermore, if the pilot signal is also used asan amplitude reference signal to automatically control the receivergain, a constant received audio signal level may be maintained.

Numerous possibilities exist for the location of the pilot signal. Thegeneration of a pilot carrier may simply be done by providing acontrolled leakage path around the transmitter sideband filter, asdescribed in U.S. Pat. No. 3,100,871. There, a pilot carrier signal issimultaneously transmitted with the SSB voice signal. The pilot carrieris detected by a phase locked loop, and also provides a reference signalfor the operation of squelch circuitry.

An alternate SSB system approach is described in the article entitled,"Improving Spectrum Efficiency with ACSB", Communications, March 1981,by P. H. Jacobs. This VHF-SSB system utilizes a pilot tone transmittedabove the voice band to provide a frequency reference for automaticfrequency control (AFC), an amplitude reference for automatic gaincontrol (AGC), and an audio subcarrier for low deviation FM tone squelchinformation. Syllabic amplitude companding improves the signal-to-noiseratio of the voice. Several problems remain with this approach. First,the sidebands of a digital data signal transmitted within the voice bandmay cause problems with frequency acquisition, since the above-bandpilot tone is used as the AFC reference and must be transmitted with thedata. Second, the narrow phase locked loop (PLL) required to demodulatethe FM tone squelch information cannot follow the very rapid amplitudeand frequency variations imposed upon a signal received in a movingvehicle. For example, multipath propagation at UHF causes fading whichoccurs at a rate of approximately 70 Hz at 840 MHz with a vehicle speedof 55 mi/hr. These amplitude and frequency variations cause severedistortion in the received speech signal as a consequence of the poorPLL tracking behavior.

A further problem with respect to 800 MHz radio systems is that offrequency stability. At 800 MHz, the 2 ppm (part-per-million) channeloscillators presently used in mobile radios could permit two adjacentchannel mobile transmitters to drift together in frequency by as much as3.5 kHz. With 5 or 6.25 kHz channel spacings, this much frequency errorwould result in a degradation in adjacent channel interference thatwould be intolerable. Unless very costly ultra-high stability (0.15 ppm)oscillators are utilized, adjacent narrowband channels nominally spaced5 or 6.25 kHz apart cannot be assigned in the same area withoutincurring a strong likelihood of mutual interference. One solution is toabandon single channel systems in favor of structured repeater systemswhich afford the opportunity to impart the high frequency stabilityrequirement of the base station to the mobile unit through the use ofAFC. If the repeater incorporates multiple trunked channels, then afurther improvement in spectrum utilization would be achieved. It isassumed that both voice and data must be communicated in such a trunkedsystem, since the organization of the system is directed by datatransmissions on a signalling channel.

Trunking is the automatic sharing of a block of communications channelsamong a large number of users. Such sharing is practical forapplications in which each user requires the communications channel foronly a small percentage of the time, and where few calls must beprocessed simultaneously. Although trunking concepts have been known andused extensively in the telephone industry and in 800 MHz FM radiosystems, e.g., U.S. Pat. No. 4,012,597, little work has been doneutilizing SSB trunking at 800 MHz because of the frequency stabilityproblems referred to above. Prior communications systems requiring AFChave employed an unmodulated master reference channel having a channelbandwidth of at least twice the bandwidth of a regular voice channel,e.g., U.S. Pat. No. 4,348,772. This approach would allow ample freespectrum space for a newly activated mobile unit to search for, find,and lock to the master reference frequency However, the utilization ofthis wideband reference channel approach contradicts the aforementionedgoal of efficient spectrum utilization.

It is believed that there presently are no multichannel narrowband(5-6.25 KHz) UHF radio systems, AM or FM, that efficiently send andreceive both analog voice and digital data signals within a dedicatedsingle channel bandwidth. The ideal system would offer substantially thesame level of performance currently enjoyed by existing FM systems--thatis: high quality voice transmissions, high speed data signallingcapabilities, and standard 2 ppm channel oscillator stability in thetransceiver unit--while improving the spectrum efficiency withnarrowband channels in the 800 MHz frequency band.

SUMMARY OF THE INVENTION

Accordingly, a general object of the present invention is to provide amethod and means for more efficient use of the 800 MHz land mobile radiospectrum through the use of an improved multichannel narrowbandcommunications system.

Another object of the present invention is to provide a narrowbandtrunked repeater radio system operating at 800 MHz in which both analogvoice and digital data signals are communicated within a dedicatedsingle narrowband channel bandwidth.

A further object of the present invention is to provide a radiocommunications system allowing for the use of standard 2 ppm channelelements in the mobiles, while only requiring a single very highstability oscillator in the base station.

Still another object of the present invention is to provide a dual-modemobile radio transceiver having receive circuitry that candemodulate--and derive AFC from--either digital FM or analog SSBmodulation.

Yet another object of the present invention is to provide a mobile radiocontroller capable of remembering the present state of the transceiver,accepting input signals derived from the received narrowband FM data onone channel, determining the proper next state of the transceiver, andoutputting the proper control signals to instruct the radio to receiveSSB-AM voice signals on another channel.

These and other objects are achieved by the present invention which,briefly described, is a single sideband communications system havingvoice transmissions sent via single sideband amplitude modulationutilizing a pilot carrier as an AFC and AGC reference, and havingdigital data signals sent via narrowband frequency modulation of a radiofrequency carrier confined to a channel having a same bandwidth as thevoice SSB modulation. The system's base station utilizes a very highstability reference oscillator, while the remote units utilize areference oscillator of moderate stability with AFC. The AFC initiallyacquires lock to the FM data channel carrier which is then demodulatedto determine the frequency of a second channel, which has the samebandwidth as the data channel and is used for subsequent voicecommunication. The AFC then acquires lock on the single sideband pilotcarrier on the voice channel. The controller incorporates a memory statefor holding AFC for the mobile transmit frequency.

In the preferred embodiment, the data channel incorporates 2400bits-per-second (bps) frequency shift keying (FSK) FM data as thesignalling format having 800 Hz deviation in a 6.25 kHz channelbandwidth. The voice channel includes a reduced pilot carrier locatedbelow the voice band for AFC and AGC reference, and an audio subcarrierlocated above the voice band having low speed data frequency modulationat approximately 80 Hz deviation within the same 6.25 kHz channelbandwidth. Utilizing this scheme, the mobile radio receivers haveimproved amplitude and frequency fading performance at 800 MHz. Hence,the communication system of the present invention provides a narrowbandhigh speed data channel containing trunked system information sent atfull peak envelope power (PEP) without a pilot carrier, and a singlesideband voice channel having a pilot carrier and a low speed datasubcarrier.

The dual-mode AFC circuit of the present invention includes AFCcircuitry which is capable of tracking the frequency of either: (a) afirst radio frequency (RF) carrier having voice signals modulated via asingle sideband amplitude modulation within a first channel bandwidth bylocking to the reduced pilot carrier transmitted with the SSB signals;or of (b) a second RF carrier having high speed data signals modulatedby narrowband frequency modulation within the designated channelbandwidth. The AFC circuit also includes control circuitry to memorizethe AFC control voltage during the time the mobile is transmitting, andto assist the acquisition of the FM data carrier by varying the AFCsweep rate on the signalling channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings, in the several figures of which likereferenced numerals identify like elements, and in which:

FIG. 1 is a block diagram of the single sideband communication systemaccording to the present invention;

FIG. 2 is a frequency spectrum diagram of the communication system ofFIG. 1 illustrating the modulation of a particular voice channel andcontrol channel according to the present invention;

FIG. 3a is a simplified block diagram of a mobile radio transceiveraccording to the present invention;

FIG. 3b is a block diagram of an alternate implementation for high speeddata modulation in the transmitter block of FIG. 3a;

FIG. 4 is a detailed block diagram of a single sideband receivercapable, of deriving AFC from either the voice or data channel accordingto the present invention;

FIG. 5 is a detailed block diagram of the AFC circuitry and the mobileradio controller of FIGS. 3a and 4;

FIG. 6 is a simplified flowchart diagram illustrating the generalsequence of operations performed by the mobile radio controller inaccordance with the practice of the present invention; and

FIGS. 7a, 7b, and 7c represent state table diagrams illustrating how thecontroller of FIG. 5 determines the next state and output signals as afunction of the present state and input signals in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the invention is a communication systemincluding both SSB voice and FM data channels in a trunked radio system.An overall system block diagram is illustrated in FIG. 1. The frequencyspectrum and channel relationships of the communications system areillustrated in FIG. 2. The basic system configuration of FIG. 1 is mostefficiently implemented in a trunked single sideband system havinggroups of transmit/receive paired voice channels, narrowband channelspacings (i.e., less than or equal to 7.5 kHz at 800 MHz), a single basestation transceiver 120, and identical multi-frequency mobiletransceivers 110 and 115.

Mobile transceiver 110 includes transmitter 111, and receiver 112.Control logic 113 controls the mobile radio operation as to frequency,synthesizer programming, AFC operation, signalling data decoding, etc.Antenna 114 is coupled to both transmitter 111 and receiver 112. In thepreferred embodiment, the mobile transceiver is designed fortwo-frequency simplex operation on a single trunked radio channel pair;although duplex operation is readily feasible if the number of channelpairs per call is doubled. The mobile transceiver is capable oftransmitting over frequency range F₁ through F₅ and capable of receivingover a frequency range F₁ +F_(T-R) through F₅ +F_(T-R). F_(T-R) is thetransmit/receive frequency spacing. Accordingly, base station 120receives frequencies F₁ -F₅ and transmits frequencies F₁ +F_(T-R)through F₅ +F_(T-R).

Base station transceiver 120 is comprised of central controller 130,base transmitter 140, base receiver 150, and base antenna 121. Centralcontroller 130 controls the channel assignments for the system.Initially, no voice messages are in progress and all mobile units areassigned to receive system control channel Y by monitoring thecontinuous stream of outbound data. To place a call, a mobiletransceiver transmits a packet of data over control channel X containingthe identity of both the called and the calling mobile unit. Thisservice request is received and processed by the central controller. Thecentral controller validates the service request, reviews the status ofits voice channels, and responds with a channel assignment to therequesting mobile (or perhaps, with a busy acknowledgment if all voicechannels are occupied) on control channel Y.

For example, assume that mobile transceiver 110 initiates a call.Transmitter 111 will send high speed data on data channel X to the basestation. When the call request is received at base station antenna 121,the received signal is bandpass filtered by receive filter 122, andcoupled to control channel X data receiver 128 via receiver multicoupler123. Upon receiving this service request, central controller 130instructs control channel Y data generator 135 to output a channelassignment to mobile 110 via transmitter coupler 134, linear poweramplifier 133, transmitter filter 132, to base antenna 121. Centralcontroller 130 also sends out a channel assignment to mobile transceiver115 for either the same voice channel as mobile transceiver 110 (i.e.,voice channel A) in a simplex system, or for a different voice channel(i.e., voice channel C) for a duplex system. The outbound FM signallingwords are generated by control channel Y data generator 135, andtransmitted on control channel Y at frequent intervals.

Upon receipt of the outbound signalling data word on data channel Y,mobile transceiver control logic 113 will immediately switch transmitter111 to voice channel A and receiver 112 to voice channel A'. As anadditional precautionary step to insure voice channel privacy, mobile110 will validate the channel assignment on voice channel A bytransmitting subaudible signalling data continuously for the duration ofthe message. This would function both as a connect indication to thecentral controller, and as an audio squelch control signal used by thebase. Furthermore, control logic 118 of mobile radio transceiver 115switches transmitter 116 to the appropriate voice channel (A or C).Central controller 130 then instructs the base transceiver to operate asa repeater station, utilizing voice channel A receiver 124 to receivetransmissions from mobile 110, and patching the audio through to theappropriate voice transmit channel (A'or C'). SSB voice is thentransmitted via the appropriate SSB generator (139 or 137) to mobiletransceiver 115.

FIG. 2 illustrates the frequency relationships between the transmittedand received voice and data channels In the preferred embodiment, asmany as five channels--four voice and one data--are multiplexed intoeach channel group. The T/R channel groups are separated by thetransmit/receive frequency spacing F_(T-R). Wide frequency separation isnecessary to minimize the complexity of filters 122 and 132 and theircounterparts in the mobile transceiver units. In the preferredembodiment, wherein F₁ equals 825 MHz, a typical transmit/receivespacing F_(T-R) would be 45 MHz.

Utilizing four SSB voice channels and one FM data channel, thecommunication system of the present invention provides the features andperformance of an FM trunked system while only occupying one-fourth ofthe frequency spectrum. Three SSB channels may be combined in a commonpower amplifier and can be placed within one 25 kHz channel assignmentwith minimal interference either to or from existing adjacent FMchannels. It is believed that five channels represent a reasonable upperboundary to the number of channels that can be combined. Cavitycombiners are not required at the base site if five channels or less areto be combined. For a standard 20 channel trunked system only fourtransmitter cavities would be required.

It is likely that 6.25 kHz rather than 5 kHz channel bandwidths willemerge as a standard for narrowband systems in the 800 MHz band. Thewider channel spacing will ease the economic and technical problemsassociated with frequency stability, tolerance, and multipathpropagation that are associated with narrowing the channel bandwidth.For example, the channel spacing of 6.25 kHz helps to minimize filteringrequirements in the receiver. Minimum filtering lowers the cost perunit, and also ensures that the receiver performance is optimal withfaded signals. This 6.25 kHz spacing also has the additional advantageof being a sub-multiple of 25 kHz, such that other narrowbandtechnologies, such as narrowband FM at 12.5 kHz and LPC speech at 6.25kHz, can be accommodated in an orderly manner.

FIG. 2 also shows the location of the control channel within the voicechannels. If a control channel containing continuous high speed FMsignalling data is used in the five channel group, the control channelshould be placed at the center of the group as illustrated. Thisarrangement minimizes the frequency spread of distortion productsproduced by linear power amplifier 133. Similarly, the centralcontroller will be directed to make voice channel assignments first tothose frequency allocations which are nearest to the control channel.

The present embodiment utilizes 2400 bps NRZ-FSK modulation withdiscriminator detection for the high speed data format. Although otherdata formats, such as GMSK were considered, NRZ-FSK is simple togenerate and detect and provides good performance results. The highspeed FM data signal is generated by frequency modulating a radiofrequency (RF) data carrier which is located 1900 Hz above thesuppressed pilot frequency reference. Optimum deviation for the FM datawas found to be approximately 800 Hz. The pilot carrier itself issuppressed in the control channel; hence, no pilot carrier istransmitted with the FSK data signal. This suppressed-carrier scheme hasthe advantages of: (1) allowing the modulation peak envelope power (PEP)to be increased; and (2) permitting the receiver discriminator, which isalready required to demodulate the data, to also be used to provide AFC.This permits faster acquisition of the base station frequency than ifAFC were obtained from a pilot carrier. Furthermore, the problems ofacquiring lock to a pilot in the presence of high speed data signal areavoided. The IF filter bandwidth is sufficiently wide to permit thediscriminator to respond to the data carrier over the entire range ofsystem frequency error. This AFC technique will be discussed in detailin conjunction with FIG. 5.

FIG. 2 further illustrates an exploded diagram of a representative voicechannel. The upper SSB voice band is generated by selecting the uppersideband of the amplitude modulated RF carrier via a band pass filter.The upper sideband modulation has a bandwidth of approximately 3500 Hzwithin the 6.25 kHz channel. The RF carrier itself is attenuated toapproximately 16 dB below PEP to provide a reference pilot which is usedin voice signal demodulation. An audio subcarrier, located above thevoice band at approximately 3460 Hz, is modulated with low speed FM datato provide subaudible signalling. This low speed data is transmittedcontinuously with the voice. In the present embodiment, the low speeddata carries tone squelch information via FSK modulation of the audiosubcarrier at approximately 80 hertz deviation. The low speed data istransmitted at approximately 17.5 dB below PEP. Hence, the total PEP isapproximately 3.0 dB above the peak voice power.

FIG. 3a illustrates a block diagram of dual-mode transceiver 300. Thetransceiver is comprised of receiver block 310, synthesizer AFC block330, transmitter block 340, and controller block 360. Transceiver 300represents the preferred embodiment of mobile transceiver 110 of FIG. 1.However, much of the circuitry of the mobile transceiver is common tobase transceiver 120, with the exception of the channel multiplexingcircuitry and the baseband feedback circuitry.

Transmitter audio source 341 includes a microphone, pre-amplifier, a 300Hz high pass filter, a +6 dB per octave preemphasis filter, and anoverdeviation amplitude clipper. Audio source 341 may also include a 2:1syllabic compressor if amplitude companded SSB is desired. The output ofaudio source 341 is applied to summing network 343 to providetransmitter voice modulation. Low speed data generator 342 includes a3.46 kHz audio subcarrier generator which is modulated with low speeddata. In the preferred embodiment, the low speed data is equivalent toCTCSS (continuous tone coded squelch system) information which modulatesthe audio subcarrier by FSK at approximately 80 Hz peak frequencydeviation. The transmitter voice signals and low speed data signals aresummed together at summing network 343 and applied to modulation switch344. Switch 344 is controlled by the data/voice (D/V) output controlsignal 364 of transceiver controller 361. In the voice mode, the audiomodulation and low speed data modulation are applied to SSB generator346. In the data mode, audio tone generator 345 produces a 1900 Hz toneto generate an exciter output signal, which is subsequently frequencymodulated by high speed data generator 353 via synthesizer 340 in orderto transmit high speed data. An alternate high speed data modulationapproach will be discussed later in FIG. 3b.

Either the voice (and low speed data) modulation or the tone generatorsignal is applied to SSB generator 346. The SSB drive signal produced bySSB generator 346 is upper sideband (USB), and is generated using theconventional balanced mixer/sideband filter approach which is known inthe art. SSB pilot generator 348 produces a 5.2 MHz pilot carriersignal. The pilot generator signal at the output of SSB generator 346 issuppressed by both the balanced mixer and by the selectivity of thesideband filter. In the data mode, pilot switch 347 opens to prevent thepilot carrier from being sent In the voice mode, the 5.2 MHz pilotcarrier from block 348 is combined with the USB drive signal from block346 in summing network 349. This SSB signal at 5.2 MHz is then upconverted to 825 MHz by mixing with a local oscillator (LO) frequencyfrom frequency synthesizer 333 in up-converter 350.

Although not shown in the simplified block diagram of FIG. 3a, the upconversion may be performed in several stages. For example, in thepreferred embodiment of base transceiver 120, the SSB voice signal,which also incorporates a low speed data subcarrier and a pilot, isup-converted to a band of frequencies at approximately 28 MHz by a localoscillator signal that is an integer multiple of the frequency ofsynthesizer reference 332. The resulting signal is subsequentlyup-converted in up convertor 350 to approximately 825 MHz by a signalfrom frequency synthesizer 333.

In the mobile, the output from mixer 350 is routed to bandpass filter351, which is designed to remove undesired mixer products. The filteredoutput signal is then applied to linear power amplifier 352. Amplifier352 should be capable of operating at a rated PEP of 15 to 20 watts with5 spurious adjacent-channel modulation products ("splatter") at least 60dB below PEP. The amplified signal from power amplifier 352 is thenapplied to transmit/receive switch 304. In the transmit mode of asimplex radio, the transmitter signal is routed to antenna 302 to betransmitted to the base station.

In the transmitter embodiment shown in FIG. 3a, the high speed data isused to directly frequency modulate both the reference oscillator andthe synthesizer VCO such that the RF output signal offset by the 1900 Hzaudio tone from tone generator 345 is frequency modulated. High speeddata generator 353 provides the 2400 bps NRZ-FSK data modulation inputto splatter filter 354. Filter 354 is a linear phase low pass filterhaving a -3 dB cutoff frequency of approximately 1200 Hz which providessplatter protection by substantially removing any harmonic contentpresent in the data. Thus, the high speed data may be generated as asquare wave, and the splatter will be substantially reduced by the lowpass filter. The filtered data is then applied to reference switch 356,along with the output of DC voltage source 355. Under control of D/Vsignal 364, switch 356 applies either the filtered high speed data orthe DC reference voltage through summing network 331 to synthesizerreference 332. In the voice or receive data mode, switch 356 appliesconstant DC voltage to summing network 331 such that synthesizerreference oscillator 332 is not modulated. In this mode, frequencysynthesizer 333 provides an unmodulated local oscillator signal to bothreceiver 310 and transmitter 340. Switch 356 remains in this voice modeuntil th transceiver controller determines that high speed data shouldbe transmitted.

FIG. 3b (drawing page 6/7) illustrates an alternate implementation forproviding high speed data modulation. In this embodiment, high speeddata generator 353 is applied to directly frequency modulate audio FSKgenerator 357 to provide data to modulation switch 344. The approachillustrated in FIG. 3b relies upon the transmitter SSB filter to containout of band emissions. On the other hand, the approach of FIG. 3a reliesupon premodulation splatter filter 354 for this task. Although thelatter approach yields a more sensitive system with less intersymbolinterference, the splatter protection is 12 to 14 dB worse with thepremodulation splatter filter. Note that no pilot carrier is transmittedwith the high speed data signal. This allows the modulation PEP to beincreased by approximately 3.0 dB. This also has other advantagespertaining to AFC acquisition, which will be described later.

Receiver block 310 of FIG. 3a illustrates a simplified version of thedual-mode receiver according to the present invention. A more detailedblock diagram and description will be provided in conjunction with thereceiver diagram of FIG. 4. The dual-mode receiver is capable ofdemodulating single sideband AM analog voice as well as narrowband FMdigital data. The transmitted 800 MHz signal is received at antenna 302,and applied to receiver 310 through transmit/receive switch 304.Preselector 311 provides frequency selectivity which reduces thespurious responses in mixer 312. The local oscillator signal fromfrequency synthesizer 333 is used to down convert the signal to theappropriate intermediate frequency (IF) by means of mixer 312. In thepreferred embodiment, the receiver is actually a triple conversionreceiver, having IF frequencies of 73.6 MHz, 5.2 MHz, and 12-16 kHz, aswill be described later. The IF signal produced at the output of mixer312 is then applied to both sideband filter 313 and pilot filter 314.Pilot filter 314, which has a bandwidth of 300 Hz, separates the pilotcarrier from the single sideband signal. Conversely, sideband filter313, having a bandwidth of approximately 3300 Hz, separates the uppersideband from the pilot signal. Filter 313 is used for data as well asvoice.

Both the upper sideband and pilot signals are applied to SSB detector317. The pilot carrier is then used as a frequency reference todemodulate the SSB voice. The demodulated audio signal is then appliedto low pass filter 321, which has a -3 dB cutoff frequency of 3000 Hz.This filter removes the low speed data subcarrier signal at 3460 Hz fromthe voice band. The voice band audio is then processed utilizingfeedforward AGC, deemphasis, amplitude expansion and squelch control inaudio processor 322.

The demodulated USB signal from SSB detector 317 is also applied tobandpass filter 318 which is centered around the audio subcarrier at3460 Hz to remove any voice signals. FM detector 319 demodulates the lowspeed data, and low speed data detector 320 provides low speed datainformation to AFC block 334.

In the voice mode, the pilot carrier signal from pilot filter 314 isrouted through switch 315 to FM detector 316. The pilot carrier is thenused as an AFC reference for the receiver. In the data mode, the highspeed data from sideband filter 313 is fed through switch 315 anddemodulated by FM detector 316. The demodulated high speed data is thenrouted to AFC 334, where it is detected to provide signalling andhandshake information to the mobile radio controller. Block 334 alsoderives an AFC control signal from the high speed data RF carrier. Thistechnique will be explained in detail in accordance with FIG. 5.

Controller block 360 generally illustrates that transceiver controller361 provides data/voice switching signal 364 and radio transceivercontrol lines 363 in response to indicator lines 362 from the receiverand AFC. Controller 360 will be described in detail in conjunction withFIGS. 6 and 7.

FIG. 4 is a detailed bock diagram of dual-mode receiver 310. FIG. 4 alsoincludes portions of synthesizer circuit 330 of FIG. 3a in order to showthe operation of the AFC circuitry. AFC circuit 444 of FIG. 4 isillustrated in detail in FIG. 5, which also shows the interface to themobile radio controller.

The receive signal from antenna 401 is applied through antenna switch402 to bandpass filter 403. Filter 403 substantially rejects thespurious responses which are inherent in the operation of first mixer404. Bandpass filter 403 also performs an attenuator function (describedlater) controlled by AGC delay block 412 such that the filter functionsas an AGC attenuator.

The 800 MHz signal from filter 403 is then applied to first mixer 404along with the local oscillator output of frequency synthesizer 407.Frequency data information 408 from the mobile radio controller is inputto frequency synthesizer 407 to determine the proper local oscillatorfrequency for each channel. The synthesizer provides low-side injectionto the first mixer. For example, if control channel F_(C) =870 MHz werebeing received in the mobile, the local oscillator frequency would be870-73.6 MHz=796.4 MHz. The principal elements of the synthesizer are aVCO, a divide-by 127/128-800 MHz prescaler, and a phase lock loop (PLL)frequency synthesizer with external charge pump. An 11.4 MHz referenceoscillator 409 is loyed as the reference frequency. Reference emposcillator 409 is controlled by VCO control voltage 410. This controlvoltage is derived from the AFC in the receive mode which keeps themobile receiver frequency locked to the ultrahigh stability base stationsignal.

The output of first mixer 404 is filtered and amplified by first IFfilter 405 at 73.6 MHz, and coupled to second mixer 406. Synthesizermultiplier 411 multiplies the 11.4 MHz reference oscillator frequency by6 in order to provide a second injection signal to second mixer 406 at68.4 MHz. The outpu of second mixer 406 at 5.2 MHz, is coupled to bothupper sideband (USB) filter 413 and pilot filter 415. As previouslydiscussed, the narrowband pilot filter separates the pilot carriersignal from the upper sideband signal. Pilot filter 415 is of a two-poleButterworth design having a total 3 dB bandwidth of ±300 Hz centered at5.2 MHz. USB filter 413 has a bandwidth dictated largely by the highspeed data signalling requirements. Since the filter bandwidth isgreater than that required for communications-quality voicetransmission, the extra bandwidth above the voice band may be used tocommunicate low speed data on an in-band audio subcarrier. In thepreferred embodiment, upper sideband filter 413 is a crystal uppersideband voice filter which is commonly used in point-to-point frequencydivision multiplex equipment.

The third local oscillator injection is provided by 5188 kHz oscillator414. This LO signal is coupled to both upper sideband mixer 416 andpilot mixer 417. The resulting output signal mixing products, which are

selected by lowpass filters 418 and 419, are the 12-16 kHz USB and 12kHz pilot signals, respectively. These signals are applied to gaincontrolled amplifiers 420 and 421. The gain controls for these two AGCamplifiers are fed from a common point. It is necessary to ensure thatthe gain of these amplifiers varies in a similar way as the controlvoltage changes so as to avoid dynamic range problems. Sufficientlyclose gain control tracking can be obtained using the two amplifiersections of a RCA CA3280 operational transconductance amplifier.

Proper SSB demodulation of voice requires that a reference carrier begenerated within 20 Hz of the true carrier frequency. If SSB is to beemployed in a commercial radio transmission system in other than the HF(3-30 MHz) band, a demodulation reference must be sent with the voiceband signal so that any frequency error can be automatically corrected.As previously noted, this demodulation reference is the single-frequencypilot signal derived from pilot filter 415. A pilot carrier systemreadily permits the use of a minimally complex demodulator that does notrestrict the voice band in any way. Furthermore, this type of SSB systemdoes not necessarily require the use of phase-locked loops, which maydegrade the performance of the receiver if subject to fast multipathfading.

The pilot carrier signal from amplifier 421 is coupled to rectifier 431,discriminator switch 440, and limiter 423. The rectifier is part of theAGC control circuitry which will be described later. Switch 440 is partof the dual-mode discriminator circuitry, and will also be describedlater. Limiter 423 amplitude limits the filtered pilot carrier signal,and uses the signal as a frequency reference with which to mix the USBsignal down to baseband audio in mixer 424. Due to the differingbandwidths of upper sideband filter 413 and pilot filter 415, there is atime delay apparent on the pilot signal envelope. To compensate for thisdelay, linear delay network 422 is inserted in the USB signal path. Inthe preferred embodiment, delay equalization is accomplished in delaynetwork 422 by means of a Reticon R5106 charge-coupled delay line whichoperates at the 12-16 kHz third IF frequency. When the envelope delaysare matched by adjustment of the delay line clock frequency, the randomFM produced by multipath signal propagation is substantially cancelledin the demodulator, leaving a pure audio tone without significantfrequency modulation at the output. Although the fade-induced frequencydisturbances are corrected by this means, amplitude variations due toinsufficient bandwidth in the feedback AGC still remain.

The feedback AGC response cannot possibly be made fast enough toeliminate the amplitude fluctuations due to 800 MHz Rayleigh fading.Feedback AGC will only maintain a constant amplitude output for deepfades when the Doppler shift is less than approximately one-tenth of the-3 dB bandwidth. The -3 dB bandwidth of the preferred embodimentreceivers is approximately 80 Hz; consequently, the feedback AGC isreally only effective up to a maximum Doppler frequency of about 8-10Hz. However, 55 mph, the maximum Doppler shift experienced by an 840 MHzcarrier is 70 Hz. Consequently, a second AGC system employingfeed-forward control is necessary to remove the substantial amplitudefluctuations that remain at the IF output.

The feedforward AGC consists of four elements: envelope detector 432;AGC enabling circuit 434; AGC control circuit 433; and AGC gain controlelement 429. Envelope detector 432 employs the full-wave rectifiedsignal from rectifier 431 so as to produce a ripple-free control signalfrom the 12 kHz pilot signal with minimum delay. This envelope-detectedsignal is applied to AGC enabling circuit 434, which disables thefeedforward AGC at weak signal levels to prevent excessive noise outputfrom the receiver. When feedforward AGC is used in a receiver withfeedback AGC having enough reserve gain to maintain both signal andnoise constant, the receiver may exhibit noise bursts much like thatthat of an FM receiver when the signal level drops. The solution to thisproblem is to limit the maximum increase in gain to 10 to 15 dB via AGCcontrol circuit 433. This limited AGC signal from block 433 is appliedto gain control element 429, which provides automatic gain correctionfor the SSB audio signal which is subject to amplitude variation due tofading. The SSB audio signal is then output at 430 to appropriate audioprocessing circuitry, which may include amplitude expansion ifcompression was used on transmit. Audio processing circuitry may alsoinclude a deemphasis network to remove audio distortion, assuming thatpreemphasis was performed in the transmitter, thereby achieving abalanced system.

The rectified pilot signal from rectifier 431 is also applied to lowpass filter 435, which defines the bandwidth of the AGC. The output ofthe lowpass filter 435 is coupled to amplitude comparator 445, whichcompares the average value of the rectified pilot signal with referencevoltage 446 and produces an AGC control voltage. The output of amplitudecomparator 445 is coupled to AGC switch 436, which is controlled bydata/voice signal 437 from AFC block 444. Switch 436 operates to controlthe AGC for the dual-mode receiver In the voice mode, the feedback AGCoperates in a normal closed-loop manner to make the average value of therectified pilot signal equal to the AGC reference voltage 446. However,in the data mode, the AGC feedback loop is opened and the AGC controlvoltage is connected to a fixed voltage V_(R) at 438, which forces thereceiver gain to be at or near its maximum value. Hence, in the datamode, dual-mode SSB receiver 310 operates much like a limiting FMreceiver, while operating with AM-AGC in the voice mode.

Feedback AGC delay circuit 412 contains control circuitry which appliesthe AGC control voltage to the first IF 405 and to bandpass filter 403only when the gain control range of amplifiers 420 and 421 is exceeded.This ensures that operation of the feedback AGC will not degrade thenoise figure of the receiver.

The baseband audio signal obtained from the output of mixer 424 iscoupled to low pass filter 428, which has a -3 dB cutoff frequency of3000 Hz. This filter removes the low speed data subcarrier signal fromteh voice band signal. The voice band signal is then coupled to AGC gaincontrol element 429.

The baseband audio signal obtained from the output of mixer 424 is alsoapplied to bandpass filter 425, centered at 3460 Hz, which separates thelow speed data subcarrier from the voice. The filtered low speed data isthen coupled to low speed FM data demodulator 426, which outputs lowspeed data 427 to control squelch operation.

Both the upper sideband signal from amplifier 420 and the pilot signalfrom amplifier 421 are applied to discriminator switch 440 to allow thereceiver to derive AFC in either the voice or data mode. In the datamode, discriminator switch 440 allows FM data from the USB filter to becoupled to limiter 441 and discriminator 442. In the voice mode, thepilot signal from pilot filter 415 is routed through switch 440 andlimiter 441 to discriminator 442. Limiter 441 functions to removeamplitude fluctuations from the discriminator which may be due to noiseor varying signal amplitude. Discriminator 442 demodulates the FM highspeed data in the data mode, and also serves as an indicator of receivedsignal frequency in either the voice or data mode. The discriminatoroutput at 443 is coupled to AFC 444, along with low speed data 427 fromlow speed data demodulator 426. AFC circuitry 444 outputs data/voicecontrol signal 437, and VCO control voltage 410. VCO control voltage 410corrects the frequency of reference oscillator 409. When the transceiveris in the transmit mode, VCO control voltage 410 is digitally stored byAFC circuit 444 such that the frequency stability of the base station issubstantially imparted to the mobile unit transmitter.

It can now be appreciated that dual-mode receiver 310 can receive bothsingle sideband amplitude modulated voice signals and narrowbandfrequency modulated data signals, the receiver acting as an SSB-AMreceiver with AGC in the first mode and as a limiting-FM receiver in thesecond. AFC is derived from the available received signal in eithermode.

Generally, the dual-mode AFC is designed to demodulate and derive AFCfrom either a received single sideband pilot carrier signal or areceived narrowband frequency modulated data signal. The AFC isinitially acquired on the data signalling channel by utilizing the highspeed FM data carrier as an AFC reference. When the receiversubsequently switches to a voice channel, fine corrections to thereceiver frequency are made by using the SSB pilot carrier as areference During the mobile transmit mode, the VCO control voltage ismemorized. This dual-mode AFC technique allows high speed data to besent in the same narrowband channel as voice, while retaining frequencylock to the high stability base station master reference frequency inboth modes of modulation.

The AFC incorporated into the mobile transceiver is a key element in theimplementation of the 800 MHz trunked SSB system of the presentinvention. The AFC characteristics strongly affect the operation ofother system functions, such as the mobile radio controller timingsequences, the operation of the SSB demodulator, and the feedforwardAGC. The basic principles and assumptions of the instant FC design areas follows:

1. The allowable frequency error of the base receiver and transmitter isless than or equal to 0.15 ppm. This represents an absolute error ofabout ±120 Hz at 816 MHz; thus, co-channel carrier beats will beinaudible due to the presence of 300 Hz audio high pass filters in thereceiver

2. The mobile transceiver unit will have an allowable frequency error ofless than or equal to 2 ppm. Standard FM transceiver componentspresently include reference elements specified to be within 2 ppm from-40° C. to +70° C. ambient temperature. This corresponds to an absoluteerror of about ±1630 Hz at 816 MHz.

3. The mobile transceiver, upon turn on, will acquire AFC on the datasignalling channel and correct the frequency of the transceiverreference oscillator such that the frequency error of the mobile iswithin ±50 Hz of the base station.

4. The mobile reference oscillator frequency determined through AFC onthe data channel is further adjusted on the SSB voice channel byacquiring AFC on the pilot carrier.

5. The mobile reference oscillator frequency determined when receivingbase transmissions will be memorized and held during the time when themobile transceiver is transmitting.

6. It is assumed that the frequency drift of the mobile referenceoscillator is small enough during the time the mobile is transmitting tobe corrected by a limited-range AFC in the base station receiver.

The AFC system of the present invention incorporates a frequencysweeping circuit. The sweep circuit is only used for initial acquisitionof the data channel on power up, after which a continuous AFC controlloop determines the frequency of the receiver. When the receiver ischanging frequency, transmitting, or when a received signal is absent,the last known frequency control voltage of the reference oscillator ismemorized and held constant. Therefore, the AFC requires a "signalpresence indication" (SPI) for proper operation. In the high speed datamode, this SPI could be the detection of a particular pattern in thehigh speed data, or of the presence of the data transmissionsthemselves. The present embodiment utilizes the latter method. In thevoice mode, SPI could be either an AGC voltage, or the detection ofcorrect low speed data on the voice channel. In the present embodiment,SPI is derived from the low speed data detector in the voice mode.

Referring now to FIG. 5, the components of AFC circuitry 444 and theirfunctions will now be described Discriminator output 443 is applied tolow pass filter 510, which has a 3 dB cutoff frequency of approximately0.27 Hz. Low pass filter 510 passes the average voltage of discriminatoroutput 443, which corresponds to the average frequency of thediscriminator input. The cutoff frequency must be low enough to limitvoltage variations due to high speed data modulation, but not so lowthat channel element drift cannot be tracked. The average frequencysignal 512 from low pass filter 510 is coupled to window comparator 515.The window comparator uses an appropriate discriminator reference fromswitch 508 to determine if the discriminator signal is within anacceptable range.

Two precise voltage references, data reference 506 and voice reference507, are supplied by discriminator reference 505. In the voice mode,discriminator reference switch 508, controlled by data/voice signal 437,couples voice discriminator reference 507 to window comparator input509. Input 509 then corresponds to the average voltage of thediscriminator output when the discriminator input is derived from an SSBpilot carrier which is properly centered in the pilot carrier filter Inthe data mode, switch 508 couples data discriminator reference 506 towindow comparator input 509. The voltage reference then corresponds tothe average voltage of the discriminator output when the discriminatorinput is derived from a data carrier properly centered in the uppersideband filter.

Window comparator 515 produces three output signals to indicate thedirection of the AFC correction needed. A high signal 516 indicates thatan upward frequency correction in the VCO is needed. A low signal 518indicates that a downward frequency correction in the VCO is needed. ONsignal 517 indicates that the receiver is on frequency. Mobile radiocontroller 500 utilizes these frequency correction indicators for theAFC whenever signal presence is indicated.

In the high speed data mode, signal presence indicator (SPI) 528 isprovided by high speed data detector 520. The function of high speeddata detector 520 is to determine whether or not high speed data ispresent in the discriminator output, thus indicating presence or absenceof signal in the data mode. Detector 520 utilizes the discriminatoroutput 443 and the data discriminator reference 506 to determine this bydetecting the presence of transitions in the discriminator output whichoccur at the high speed data clock frequency Another embodiment of highspeed data detector 520 would detect the presence of a particularrepeating pattern in the high speed data. Squelch detector 525 utilizeslow speed data 427 which is sent with the voice to determine whether ornot a signal is present in the voice mode by detecting the presence of aparticular pattern of coded data. In either mode, an SPI indication iscoupled by SPI switch 527 as controlled by data/voice signal 437. Calldetector 522 provides a call indication signal 523 to the mobile radiocontroller whenever a transceiver unit is being paged by the centralcontroller at the base station. The call detector operates by detectingthe occurrence of a particular pattern of coded data.

Upon power up, the synthesizer is programmed to go to the high speeddata signalling channel. The AFC initially forces the frequencyreference in the transceiver to sweep ±2 kHz at a fast rate of 2kHz/second At some point in the sweep cycle, the frequency error of thetransceiver reference (minus the minor error in the base) will becompensated. The receiver will then recognize that 2400 bps data isbeing received. The AFC sweep rate is then slowed to 100 Hz/second forfinal centering on a ±50 Hz window. If the frequency is within theselimits, the AFC is considered to be "on frequency." The informationderived from the high speed data is coupled to mobile radio controller500 to control the operation of the transceiver. The transceiver is theninstructed to move to a voice channel to either place or receive a call.Once on the voice channel, the received pilot carrier is used as the AFCsignal to which the transceiver reference can be adjusted The presenceof valid low speed data on the voice channel indicates to the controllerthat a valid pilot is being used by the AFC circuitry. In the absence oflow speed data, the transceiver frequency reference is held.

The fast AFC sweep that occurs at power up can be described as theopen-loop mode of the AFC. The sweep is uni-directional, and isterminated by the SPI from the high speed data detector. The SPIcircuitry operates in the full USB filter bandwidth, but the frequencywindow over which the high speed data can be recognized, which is afunction of the amount of distortion caused in the USB filter, isapproximately ±220 Hz. The response time of the SPI is determined by itsown bandpass filter, and is 220 milliseconds under worse caseconditions.

The AFC operation changes from open loop to closed-loop operation assoon as the SPI goes high. The sweep rate is also reduced to aid infrequency acquisition. The incoming data signal is frequency demodulatedand heavily filtered in lowpass filer 510 in order to determine avoltage corresponding to the average frequency. A comparison of thisvoltage to the fixed voltage provides a signal which directs thefrequency reference accordingly. When in the closed-loop mode, the AFCmust reduce the 220 Hz possible overshoot error incurred in theopen-loop mode to less than ±50 Hz. This should be done as quickly aspossible. However, the faster the slow sweep rate is, the higher thecutoff frequency of lowpass filter 510 must be in order to avoid anunderdamped loop. If lowpass filter 510 is too wide, the random bitpattern will cause the "on frequency" voltage to vary excessively. Afilter cutoff frequency of 0.27 Hz is used in the present embodiment,and the AFC slow sweep speed employed for use with this filter is 100Hz/second. This narrow filter bandwidth is necessary in order to reducethe variations in average frequency due to the random FM present whenthe received signal is fading.

AFC operation on a voice channel is essentially the same as for theclosed-loop system described above. The same loop filter and slow sweepspeed that were used on the data channel are used on the voice channel.However, now the "on frequency" reference voltage is lower, since thepilot carrier frequency is 1900 Hz below the center frequency ofthe highspeed data. The voltage reference is switched accordingly. The audiosubcarrier with low speed data is demodulated and the data is detectedto determine whether the AFC should be allowed to adjust the transceiverfrequency reference. The frequency reference is held constant in theabsence of a proper low speed data code, and the system controller waitsfor either a valid code or a time-out indication to return to the datachannel. The fast sweep mode is never used on the voice channel

The discriminator used is a delay-line discriminator in the 12 kHz IF,which is implemented using a Motorola MC14562B shift register Thediscriminator constant is 1 mv/Hz. The clock for the discriminator isderived from a crystal oscillator, so the signal delay is extremelystable and should not contribute significantly to frequency errors.

Discriminaor reference 505 is based on the Motorola TL431 programmablevoltage reference IC. The nominal voltages of the references are 3.75volts DC for voice reference 507, and 5.65 volts DC for the datareference 506. Low pass filter 510 is comprised of a single pole RCcircuit having a 0.27 Hz cutoff frequency. This very low cut offfrequency requires large component values. In the preferred embodiment,the resistor is 150 kilohms, and the capacitor is 3.9 uF. To facilitateswitching between voice and data channels, one end of the capacitor istied to the switched discriminator reference 509.

The ±50 Hz frequency window defined by window comparator 515 is producedby two sections of an LM339 quad comparator which are biased to switchwhen the discriminator voltage deviates by more than 50 mV from theswitched discriminator reference 509. A NOR gate is employed to createthe ON (frequency) signal 517 from the output signals of the twocomparators.

High speed data detector 520 is composed of a limiting amplifier, twoedge detectors, a commutating bandpass filter, a rectifier, and athreshold detector with a fast rise time and slow fall time. Pulsesgenerated at the rising and falling edges of the demodulated high speeddata input from discriminator output 443 provide significant energy at2400 Hz, which is extracted by a bandpass filter having a bandwidth of30 Hz. The rise time constant of the threshold detector is 11 ms, andthe fall time constant is 1.5 seconds. This arrangement provides a greatdeal of fade protection for the signal presence indicator (SPI) output.Call detector 522 is in parallel with high speed data detector 520, andis of a similar design An alternate embodiment of a call detector wouldbe a correlation detector. Call detector 522 may also be incorporated aspart of mobile radio controller 500.

The operation of mobile radio controller 500 is described in detail inthe following figures. In general, mobile radio controller 500 remembersthe present state of the mobile, accepts information from the radiotransceiver circuitry, determines the proper next state based on theseinput signals and the present state, and then produces correspondingoutput control signals. The input/output lines 551-559 are particular tothe radio transceiver used in the preferred embodiment, and areillustrated in FIG. 5 to better describe the function of the state tablediagrams of FIG. 7.

Up/down counter 530 utilizes up/down control signal 531 and HOLD controlsignal 532 from mobile radio controller 500 to produce digital outputsignals 534. Digital-to-analog converter 535 changes digital outputsignals to the required analog VCO control voltage 410. Two speed clock540 controls the slope of the VCO control voltage-versus-timecharacteristic. For initial AFC acquisition, a fast sweep speed ofapproximately ±2 kHz/second is used. When the presence of high speeddata is detected by high speed data detector 520, slow/fast clock line541 instructs two speed clock 540 via clock line 542 to slow a secondspeed of 100 Hz/second which allows frequency centering in the desiredwindow. As previously described, the significant reduction in sweepspeed is necessary because the discriminator output is processed by lowpass filter 510 which has a very low cutoff frequency. The slow speed isalso used in the voice mode, since the VCO control line voltage is knownto be within the ±50 Hz window when the voice mode is entered. Duringthe transmit mode, HOLD control line 532 halts the operation of up/downcounter 530, such that the VCO control voltage 410 is memorized. Thishold operation also takes place during the switching time between thedata and voice modes.

In the preferred embodiment, up/down counter 530 is an 8-bit up/downcounter formed from two Motorola MC14515B 4-bit counters. The counter isunidirectional when in the fast-sweep or open-loop mode, andbi-directional in the slow-sweep or closed-loop mode. Two-speed clock540 is implemented using a Motorola MC1555 timer and a switched timingcapacitor. D/A converter 535 is a Motorola MC1408 digital-to-analogconverter.

Mobile radio controller 500 outputs frequency data 408 to frequencysynthesizer 407 (FIG. 4) to determine the correct receive/transmitchannels. PTT input line 556 indicates a push-to-talk transmit requestto the controller. T/R output line 551 and A³⁰ T output Line 553 providetransmit/receive switching controls to the radio circuitry. EOT(end-of-transmission) input line 558 is used in conjunction withtransmitting low speed data. Mute output line 554 is used to control theradio squelch circuitry. Timer output line 555, and time-out-timer (TOT)input line 557, are used with an external timer circuit. Reset line 559is used with external power-up-reset circuitry. Beep output line 552controls an indication to the user that the system is busy. These inputsand outputs of mobile radio controller 500, among others, will bedescribed in detail in conjunction with the following figures.

FIG. 6 is a flowchart representing the sequence of operations performedby mobile radio controller 500. Mobile controller 500 is a state machinedesigned to output the proper control signals based upon presentreceiver input signals and past history. The mobile controller of thepreferred embodiment has seven input signals, twelve states, and eightoutput signals. Each of these states and output signals will bedescribed in detail in conjunction with the state table diagrams of FIG.7.

As illustrated in FIG. 6, the mobile radio controller begins operationat start block 601, and proceeds to block 602 wherein power-up-reset ofthe mobile transceiver is caused by external reset control signal 559.As indicated in block 603, the present state of the controller isinitially set equal to state zero, which is the power-up or reset state.In state zero, as indicated by block 604, the frequency synthesizer isprogrammed to receive the data channel. In block 605, external controlsignals are accepted by simultaneously clocking all seven input signalsinto the controller. These input signals, such as reset, signal presenceindicator, AFC on frequency, time out timer, etc., are then utilized todetermine the next controller state and corresponding output controlsignals as a function of the controller's present state and these inputsignals. This step is performed in block 606 by utilizing the statetables programmed into the controller's memory. The state tables for thepreferred embodiment are contained in the Appendix. An alternateapproach would be to calculate the next state and output signals throughthe use of a computer algorithm. However, this approach was not asefficient in the preferred embodiment as the instant approach of a ROMlook-up table.

After the next state and output signals are determined, the outputcontrol signals are sent to the radio transceiver in block 607.Depending on which state has been chosen as the next state, thefrequency synthesizer may or may not have to be re-programmed to receiveor transmit a different channel. This decision is made in block 608. If,for example, the previous state was the power-up state where the radiois receiving the data channel, and the next state is the transmit state,the frequency synthesizer would have to be programmed to the assignedvoice channel in block 609. On the other hand, if high speed data hasbeen detected but a different mobile has been called, then the mobileremains on the data channel in the receive mode. In the latter case, thesynthesizer reprogramming sequence of block 609 is bypassed andoperation continues with block 610. The mobile radio controller updatesthe present state to the next state in block 610. The controller remainsin the present state until new input signals are clocked into thecontroller in block 605. Hence, the controller of the present inventionfunctions as a state machine utilizing the present state and inputsignals to determine the controller's next state.

In the present embodiment, the seven controller inputs and twelvecontroller states are organized as the eleven address bits each of two8-bit×2K EPROMS; the seven least significant bits directly rpresent theinputs to the controller, the remaining four most significant bitsrepresent a binary code for one of the twelve present states of thecontroller. D flip-flops on all EPROM inputs, with the exception of theRESET line, prevent feedback paths through the transceiver from changinginputs within a controller cycle. The controller clock rate of 44 Hz islimited primarily by the switching time for the synthesizer. Four morestates could readily be implemented if desired. Four bits of each of the8-bit words stored in one of the EPROMS are used as the next stateoutput. The next state becomes the present state after one controllerclock cycle.

Again, in the present embodiment, it is necessary to provide eightindividual output lines to control the functions of the transceiver.Accordingly, the second 2K EPROM is employed to provide the propercombination of the eight output control signals. The eleven address bitsof the second 8-bit×2K EPROM are organized in the same manner as thenext state EPROM. The 8 bits of each word stored in the second EPROM arethen used as outputs to directly control each of the transceiverfunctions.

The partial state table diagrams of FIG. 7 illustrate how controller 500performs the state table look-up function of block 606. The actual nextstate table and output code table of the present embodiment are includedin Appendix A and Appendix B, respectively.

FIG. 7a illustrates the basic method of reading the state tablediagrams. The 12 present states are arranged on the left side of thestate table in ascending order from the top of the vertical axis to thebottom. The input signal code combinations are arranged in ascendingorder from left to right across the top of the horizontal axis. Sincethe controller uses seven input lines to determine the next state, 128possibilities exist (2⁷) for the input code. The controller utilizes thepresent state axis and the input code axis as addresses to determine theappropriate next state and output code.

To facilitate faster input code addressing and more efficient use ofEPROM memory, the state tables of the preferred embodiment are arrangedin a slightly different manner then shown in FIG. 7a. Two state tables,one represents the next state EPROM, and another for the output codeEPROM, are utilized. FIG. 7b illustrates the arrangement of the nextstate table, which will be found as Appendix A. Each present state hasbeen divided into seven rows and fifteen columns of input signals. Thefirst row corresponds to binary input codes of 0-15, the second rowcorresponds to binary input codes 16 through 31, etc. The seventh rowcorresponds to binary input codes 112 through 127 Hence, the 128 inputcode possibilities for present state zero have been arranged to morequickly address the next state. The same input code addressingarrangement is duplicated for present state one. As may be seen fromAppendix A, the next state information is available from the first EPROMin decimal numbers (0-11).

FIG. 7c illustrates the addressing scheme for the output code EPROMstate table found as Appendix B. The addressing scheme is exactly thesame for the output code table as that of the next state tablepreviously described in FIG. 7b. However, the output code information isgiven in hexadecimal notation (00-FF). Hence, there are 256 output codepossibilities for the eight output signal lines.

An example of utilizing the state tables of the appendix is given below.Assume that the mobile radio has just been turned on, such that thepresent state is STATE 0. Furthermore, assume that the referenceoscillator 409 has been swept to a frequency such that the receivedsignal falls within the response range of the signal presence indicator(SPI) 528, but does not fall within the response range of the ONindicator 517. Also, assume that the mobile user has pressed themicrophone PTT switch. With these assumptions, PTT 556 and SPI 528 arehigh, and the rest of the input signals are low. This input combinationwould represent a binary 0000110 which is equivalent to decimal 6. Todetermine the next state, the controller utilizes this binary input codeand present state information to address the next state table ofAppendix A. The next state may be found to be STATE 4, by reading 4 fromthe first row and seventh column of present STATE 0. This addresslocation corresponds to binary input code 6. Similarly, the output codemay be determined by reading the first row, seventh column of presentSTATE 0 in Appendix B to find hexadecimal 04. This output code tells thecontroller to change output line slow/fast (S/F) 541 to the "slow" clockcondition and to deactivate all other output lines. The controlleroutputs this information and changes the present state to STATE 4.

An alternative method for arriving at the proper level in the tables ofAppendix A and Appendix B is to convert the input binary code tohexadecimal code, and then use the first hexadecimal digit to calculatethe row number within the entries for the proper present state, and usethe second hexadecimal digit to calculate the column number. Forexample, input hexadecimal code MN corresponds to the entry in the(M+1)th row and the (N+1)th column of the entries for any particularpresent state.

Continuing with the example above, assume that the controller is inSTATE 4. Also assume that the mobile user is still pressing themicrophone PTT switch. Furthermore, assume that the frequency ofreference oscillator 409 has been corrected such that the signal fallswithin the response range of the ON indicator 517. However, also assumethat the mobile user has driven out of range of the base transceiversignal such that the SPI indication 528 is low. Under these conditions,the input word is binary 0000101 which is equivalent to hexadecimal 05.The next state is found in the first row and sixth column of presentSTATE 4 in Appendix A--next STATE 2. Similarly, the output code fromAppendix B is found to be hexidecimal 4C. Thus, beep output line 552,timer output line 555, and slow/fast (S/F) output line 541 areactivated, and all other outputs are deactivated The controller thenadvances to STATE 2.

The activation of timer output line 555 causes, the activation of timeout timer (TOT) input line 557 due to circuitry external to thecontroller. Assuming that no other input lines change as the controlleradvances to STATE 2, the input code will then be binary 0010101 which isequivalent to hexadecimal 15. The second row and sixth column of presentSTATE 2 in Appendix A indicates that the controller is to remain inSTATE 2. Similarly, the hexadecimal output code 44 may be obtained fromAppendix B.

The seven input signals corresponding to the binary input code arelisted below, in order, from most significant bit (msb) to leastsignificant bit (lsb).

1. RESET - Input line 559, from either a mechanical switch or externalcircuitry, tells the controller to return to the power up state.

2. EOT (End-Of-Transmission) - Input line 558 from the low speed datacircuitry tells the controller that the low speed dataend-of-transmission code has been completed.

3. TOT (Time-Out-Timer) - Input line 557 from external time-out-timercircuitry is used to mask a momentary loss of the signal presenceindicator.

4. CALL - Input line 523 from call detector 522 operating on the highspeed data, indicates that the present mobile is being called.

5. PTT (Push-To-Talk) - The transmit request line 556 derived from themicrophone PTT button.

6. SPI (Signal Presence Indicator) - Line 528 from squelch detector 525or high speed data detector 520, indicates that 2400 bps data is presenton the data channel or that the proper low speed code has been detectedon the voice channel.

7. ON (On Frequency) - Indicator line 517 from window comparator 515,tells the controller that the receiver is within a ±50 Hz window of theproper frequency on either the data channel or the voice channel.

The eight controller output signals are listed below in the same(msb-lsb) order:

1. A⁺ T - Output line 553 controls the transmitter final power amplifierkeying.

2. BEEP - Output line 552 provides an indication to the mobile user thathis PTT signal is being ignored due to lack of an available transmitchannel.

3. MUTE - Output line 554 indicates to the receiver squelch circuitrythat it should mute the audio.

4. HOLD - Output line 532 to up/down counter 530 indicates that VCOcontrol voltage 410 should be held constant.

5. TIMER - Output line 555 initializes the time-out-timer circuitry.

6. (Slow/Fast) - Output line 541 to two speed clock 540 controls the AFCcircuitry by indicating the correct speed at which to sweep the VCOfrequency.

7. T/R (Transmit/Receive) - Output line 551 directs the synthesizer, theantenna relay, and the majority of the transceiver circuitry to changefrom transmit to receive mode.

8. D/V (Data/Voice) - Output line 437 directs the synthesizer and otherswitching circuitry to change between the data and the voice channels.

The twelve states of the mobile radio controller (STATE 0-STATE 11) aredescribed below:

STATE 0--the power-up or reset state. As previously mentioned, this putsthe transceiver in the receive mode on the data channel. This state isalso entered when the mobile drives out of range of the base station.The synthesizer reference frequency is being swept at its fast sweeprate of 2 kHz/second in order to try to acquire AFC on the data channel.PTT will be ignored and cause a beep signal. The controller leaves thisstate when signal presence is detected.

STATE 1--In this state, the signal presence indicator (SPI) is high.However, the synthesizer reference frequency error is greater than 50Hz. Therefore, the synthesizer reference frequency is being changed atthe slow rate of 100 Hz/second for frequency centering PTT and CALL willbe recognized, but will not be acted upon until ON is high. If SPIshould go low, the time-out-timer will be initialized and STATE 2 willbe entered.

STATE 2--In this state, SPI is low, but the synthesizer referencefrequency is being held constant just in case the drop in signal is onlymomentary. A time-out indication will return the controller to STATE 0.A high SPI signal will change the state of the controller to STATE 1, 3,4, 5, 6, or 7 according to the other input signals.

STATE 3--SPI and ON are high, such that the synthesizer referencefrequency is being held constant. However, neither PTT nor CALL is high,so the controller keeps the mobile transceiver on the data channel inthe receive mode waiting for a call or a transmit request.

STATE 4--Here, both PTT and SPI are high, but the synthesizer referencefrequency is still off frequency, i.e., ON is low. The synthesizerreference frequency is being changed at the slow rate.

STATE 5--This is the transmit state. This state is entered when PTT,SPI, and ON are all high and CALL is low. As this state is entered, T/Ris set high in order to activate the modulation and transmit circuitry.The data/voice signal is set to "voice" to change the mobile from the FMdata mode to the SSB voice mode. Once in this state, A⁺ T is set highwhich indirectly activates the final power amplifier. PTT and RESET arethe only inputs which have an effect on this state. When PTT goes low,the controller advances to STATE 11.

STATE 6--In this state, both CALL and SPI are high, but ON is still lowsince the synthesizer reference frequency is still off frequency Hence,the synthesizer reference frequency is being changed at the slow sweeprate waiting for ON to go high.

STATE 7--This state is a transition state from the data to the voicechannel. STATE 7 is entered when CALL, SPI and ON are all high. As thisstate is entered, the time-out-timer is initialized, and the synthesizerreference frequency is held constant. The data/voice signal is changedto the voice mode. If the voice channel SPI does not go high before thetime-out period expires, the controller will return to STATE 2; i.e.,the mobile will return to the data channel. If the voice channel SPI isrecognized, the controller proceeds to STATE 9 or 10. In STATE 7, PTT isignored, since the mobile was called to the voice channel and a lack ofa recognizable signal may indicate the mobile has made some kind oferror.

STATE 8--This is a waiting state and is similar to STATE 7. Thetime-out-timer is initialized when this state is entered, and a time-outcondition will cause the controller to return to STATE 2. However,before STATE 8 is entered, either the mobile has received the validvoice channel signal, or it has made a transmission on the voicechannel. For this reason, PTT is recognized and the controller isadvanced to STATE 5. SPI will advance the controller either to STATE 9or to STATE 10.

STATE 9--This is one of two states for which the mobile is in thereceive mode with the audio squelch open In this state, the voicechannel SPI is high, but ON is low. The synthesizer reference frequencyis changed slowly to correct for small frequency errors. The controllerleaves this state for STATE 10 when ON goes high, or the controller goesto STATE 8 when SPI goes low.

STATE 10--This is the other of the two states for which the mobile is inthe receive mode with the audio squelch open. In this state, both thevoice channel SPI and ON signals are high. Thus, the synthesizerreference frequency is held constant. This state is exited in a mannersimilar to STATE 9.

STATE 11--This is an interim state between STATE 5 and STATE 8. When themobile user releases PTT, this state is entered from STATE 5, and the A⁺T signal is dropped. This transition causes the low speed data encodingcircuitry to output the turnoff code. When the low speed turnoff code iscompleted, the EOT signal input goes low. This change in EOT causes thecontroller to go to STATE 8. The EOT signal itself shuts down thetransmitter power amplifier. The controller will change the T/R signalback to the receive mode once STATE 8 is reached. This has the effect ofdelaying the T/R change from the EOT change, which prevents the antennarelay from opening before the power amplifier has turned off, as well aspreventing transmission of erroneous signals as the modulation circuitryis being switched by T/R.

As a final example of the operation of the present invention, a completecall sequence operation will now be described.

1. Central controller 130 has established base transmit channel Y andbase receive channel X as control channels. The base station operates inthe full duplex, non-repeat mode with respect to the control channels.

2. Central controller 130 sends out a continuous background data word ondata channel Y. The background data word is a 2400 bps decodable datapattern that allows the mobiles to maintain synchronization in thetrunked system.

3. On power-up, mobile radio controller 500 of mobile transceiver 110 isput in STATE 0. Receiver 112 is activated by T/R output line 551.Synthesizer frequency data 408 changes frequency synthesizer 407 suchthat receiver 112 accepts the signal of base transmit channel Y. Up/downcounter 530 is forced to count in one direction by U/D output line 531.Up/down counter 530 counts at the fast speed in accordance with clockline 542. VCO control voltage 410, derived from digital output signals534, causes synthesizer reference 409 to sweep frequency in such amanner that the received signal of base transmit channel Y (astranslated by mixer 404 and mixer 406) moves in frequency across thepassband of upper sideband filter 413. Discriminator 442, operating on afrequency-translated version of this signal, passes the baseband signalto high speed data detector 520. When the receiver injection signal,derived from reference oscillator 409, is near the correct frequency,high speed data detector 520 indicates the presence of the 2400 bpsbackground data word to mobile radio controller 500 via SPI input line528.

4. The change of SPI input line 528 causes mobile radio controller 500to change to STATE 1. Simultaneously, U/D output line 531 is changedaccording to the signals on HIGH input line 516 and LOW input line 518.S/F output line 541 is also changed such that two speed clock 540operates at its slower speed. VCO control voltage 410 adjusts thefrequency of reference oscillator 409 such that the continuousbackground data word on data channel Y (as translated by mixer 404,mixer 406, mixer 416, and discriminator 442) has an average voltageapproximating that of data discriminator reference 506. Once the averagevoltage is within an acceptable window of reference 506 as determined bywindow comparator 515, ON input line 517 is activated.

5. The change of ON input line 517 causes mobile radio controller 500 tochange to STATE 3. Simultaneously, HOLD output line 532 is activatedsuch that up/down counter 530, digital output signal 534, VCO controlvoltage 410, and reference oscillator 409, are all held constant.

6. Mobile transceiver 115 also powers-up in STATE 0 and arrives in STATE3 in a manner similar to steps 3 through 5 above for mobile transceiver110. Mobile transceivers 110 and 115 both decode the 2400 bps backgrounddata word of base transmit channel Y.

7. The user of moblle transceiver 115 initiates a call by pressing themicrophone PTT button of that transceiver. Recognizing the change on PTTinput line 556, mobile radio controller 500 of transceiver 115 changesto STATE 5. Simultaneously, T/R output line 551 and D/V output line 437of mobile transceiver 115 change value such that transmitter 116 (exceptlinear power amplifier 352) is activated, and receiver 117 isde-activated. When the proper transmit channel is determined, A⁺ Toutput line 553 of mobile transceiver 115 is activated. This causespower to be supplied to linear power amplifier 352 of mobile transmitter116.

In the preferred embodiment of a fully trunked SSB communication system,a "high speed handshake" takes place between the mobile requesting achannel and the base station controller. This handshake sequence isdescribed as substeps a-d below. For simplicity, however, the statetable diagrams included in the Appendix do not contain this handshakeinformation. The state table diagrams of the Appendix continue with Step8 below.

(a) The "high speed handshake" begins when the user of mobiletransceiver 115 initiates a service request by pressing the microphonePTT button. Mobile 115 then transmits an inbound high speed datasignalling word on base receive control channel X. This inboundsignalling word contains mobile, subfleet, fleet, and systemidentification information along with the type of call.

(b) When base station controller 130 receives this inbound signallingrequest, it will automatically review the present channel assignmentsand queueing list. If the system is not full, base transceiver 120 willthen transmit an outbound signalling word on base transmit controlchannel Y. This outbound signalling word contains channel assignmentinformation (in the event that a channel is available) or busy statusinformation (in the event that a channel is unavailable).

(c) Upon receipt of the outbound signalling word via control channel Y,all mobiles in the particular fleet switch to the assigned voice channele.g. voice channel A/A'.

(d) The appropriate mobile units, now receiving assigned voice channelA', decode the high speed handshake information. The mobile thatinitiated the call, mobile 115, then sends a high speed dataacknowledgement signal back to base transceiver 120 via mobile transmitvoice channel A. This acknowledgement signal verifies to the basestation that the mobile is on the correct channel

8. In STATE 5, transmitter 116 transmits a single sideband pilot carrierand low speed FM data on base receive voice channel A via antenna 114.This signal is received by base receiver 150 via antenna 121 anddemodulated by voice channel A SSB receiver 124. Central controller 130recognizes the low speed data code, changes the high speed data patternbeing transmitted on base transmit control channel Y from the backgrounddata pattern (or outbound signalling word) to a specific call sequence,and instructs base transmitter 140 to transmit a pilot carrier and lowspeed data pattern on base transmit channel A'.

9. Mobile receiver 112 receives the high speed data pattern beingtransmitted on base transmit control channel Y. Call detector 522 ofmobile receiver 112 recognizes the call sequence being transmitted andchanges CALL input line 523 to indicate a valid detection to controller500 of transceiver 110. Mobile radio controller 500, which was in STATE3, is changed to STATE 7. Simultaneously, TIMER output line 555 ischanged to initialize the time-out-timer, and D/V output line 437 ischanged such that mobile transceiver 110 operates in the voice mode.Synthesizer frequency data 408 of mobile transceiver 110 is also changedsuch that mobile receiver 112 receives base transmit voice channel A'.

10. In the voice mode, low speed data 427 from low speed datademodulator 426 of mobile transceiver 110 is recognized as the propercode by squelch detector 525. SPI input line 528, which was low duringthe transition to the voice channel, is set high to indicate thepresence of the voice signal on base transmit channel A'. This SPIchange causes controller 500 of transceiver 110 to change to STATE 10.Simultaneously, MUTE output line 554 is changed to allow the audiosignal of receiver 112 to be heard by its user.

11. The user of transceiver 115 now talks into the microphone of thatradio. This voice signal is transmitted via single sideband modulationon base receive voice channel A from mobile transmitter 116, and isreceived and demodulated by voice channel A SSB receiver 124 of basereceiver 150. Central controller 130 then patches this voice signalthrough to modulate voice channel A SSB generator 139 of basetransmitter 140. The SSB voice signal is then transmitted on basetransmit voice channel A' and received and demodulated by receiver 112.This completes the repeater link between mobile transceiver 115 andmobile transceiver 110.

12. When the user of mobile transceiver 115 has finished his voicecommunication, he releases the PTT switch for that unit. The change onPTT input line 556 of transceiver 115 causes its controller 500 toadvance to STATE 11. Simultaneously, A⁺ T output line 553 is set low.This causes low speed data generator 342 of mobile transmitter 116 tooutput a "turn-off" code while holding EOT input line 558 high. Thisturn-off code is recognized by central controller 130, which thendirects base transmitter 140 to re-transmit the turn-off code on basetransmit channel A'. Subsequently, the SSB signal generated by voicechannel A SSB generator 139 is stopped. The high speed data signal beingtransmitted by base transmitter 140 on base transmit control channel Yis then changed from the specific call sequence to the background dataword.

13. Once the turn-off code of mobile transceiver 115 has been completed,EOT input line 558 of mobile transceiver 115 is set low. This causescontroller 500 of mobile transceiver 115 to advance to STATE 8, and alsoremoves supply power from linear power amplifier 352. Simultaneously,TIMER output line 555 is set high to initialize the time-out-timer. Thiscauses TOT input line 557 to go high. Once in STATE 8, T/R output line551 is changed to force transmit/receive switch 304 back to the receiveposition, to activate mobile receiver 117, and to de-activate mobiletransmitter 116. Also, synthesizer frequency data 408 of transceiver 115is changed such that mobile receiver 117 again receives base transmitvoice channel A'.

14. When the turn-off code is received by mobile receiver 112 andrecognized by squelch detector 525 of that unit, SPI input line 528 willbe set low. Controller 500 of mobile transceiver 110, which previouslywas in STATE 10, advances to STATE 8. Simultaneously, MUTE output line554 is set low such that the audio signal of that unit is squelched, andTIMER output line 555 is set high to initiate the time-out-timer ofmobile transceiver 110. Accordingly, TOT input line 557 goes high.

15. At this point, both mobile transceivers 110 and 115 are in STATE 8.Either one may initiate a call to the other. However, if neitherinitiates a call within a specified amount of time (as determined by thetime-out-timer), TOT input line 557 of each unit will go low and thecontroller of each unit will return to STATE 2. Simultaneously, D/Voutput line 437 of each unit is changed such that both mobiletransceivers return to the data mode. TIMER output line 555 of each unitis again set high to initialize their time-out-timers. Synthesizerfrequency data 408 of each unit is also changed such that mobilereceivers 112 and 117 receive base transmit control channel Y.

16. If mobile transceiver 115 remains within range of base transmitter140, high speed data detector 520 of transceiver 115 will recognize thepresence of the background data word received on control channel Y. ThisSPI signal will cause controller 500 of mobile transceiver 115 toadvance either to STATE 3 (if reference oscillator 409 has not driftedoutside the acceptable "ON" window), or to STATE 1 (if referenceoscillator 409 has drifted outside this window.)

17. In the event that mobile transceiver 115 is no longer within therange of base transmitter 140, high speed data detector 520 of mobiletransceiver 115 will not recognize the presence of the background dataword. After an appropriate amount of time, as determined by thetime-out-timer, TOT input line 557 of transceiver 115 will go low andcontroller 500 will advance to STATE 0. Simultaneouly, output HOLD line532 and S/F output line 541 will be changed such that VCO controlvoltage 410 is swept at the faster rate as it was on power-up.

In review, an SSB communication system with FM data capability has beendescribed to provide more efficient use of the 800 MHz land mobile radiospectrum. The base station transceiver transmits voice signals viaSSB-AM in a specified voice channel, and transmits high speed datasignals via narrowband FM in a specified data channel having a bandwidthno wider than that of the voice channel. Each remote station transceiverof the system includes a dual-mode receiver capable of receiving eitherthe SSB-AM voice signals or the high speed FM data signals in responseto information derived from the high speed data signals. Depending onthe system configuration, the voice and data signals may be assigned todifferent channels, or may be assigned to the same channel.

While only particular embodiments of the invention have been shown anddescribed herein, it will be obvious that certain modifications may bemade without departing from the invention in its broader aspects and,accordingly, the appended claims are intended to cover all such changesand alternative constructions that fall with the true scope and spiritof the invention. ##SPC1##

What is claimed is:
 1. A frequency generating apparauts for a radioreceiver having means for receiving voice and data signals over any oneof a plurality of radio frequency (RF) channels in response to areceiver injection signal, said frequency generating apparatuscomprising:local oscillator means for generating said receiver injectionsignal responsive to an automatic frquency control (AFC) signal andresponsive to a channel selection signal; control means for providingsaid channel selection signal in response to information derived from afirst type of modulation received on a first radio frequency (RF)carrier within a first channel of said plurality of channels, said firstchannel having a predetermined bandwidth; and circuit means forproviding said AFC signal by tracking the frequency of one of either:(a)said first received RF carrier having said first type of modulationwithin said first channel bandwidth; and (b) a second received RFcarrier having a second type of modulation within a second channelhaving a predetermined bandwidth being no wider than said first channelbandwidth.
 2. The frequency generating apparatus according to claim 1,wherein said second type of modulation includes voice signalstransmitted via single sideband (SSB) amplitude modulation (AM).
 3. Thefrequency generating apparatus according to claim 1, wherein said firsttype of modulation includes data messages transmitted via narrowbandfrequency modulation (FM).
 4. The frequency generating apparatusaccording to claim 2, wherein said SSB-AM includes a reduced pilotcarrier
 5. The frequency generating apparatus according to claim 4,wherein the amplitude of said pilot carrier is approximately 16 dB belowpeak envelope power.
 6. The frequency generating apparatus according toclaim 2, wherein the peak amplitude of said voice signals isapproximately 3.0 dB below peak envelope power.
 7. The frequencygenerating apparatus according to claim 3, wherein said FM istransmitted without a pilot carrier.
 8. The frequency generatingapparatus according to claim 3, wherein said FM is transmitted at fullpeak envelope power.
 9. The frequency generating apparatus according toclaim 3, wherein said data messages are centered at approximately 1.9kHz above the lower edge frequency of said first channel.
 10. Thefrequency generating apparatus according to claim 3, wherein said datamessages are digital data modulation at approximately 2400 bps.
 11. Thefrequency generating apparatus according to claim 3, wherein thefrequency deviation of said data modulation is approximately 800 Hz. 12.The frequency generating apparatus according to claim 1, wherein saidfirst channel bandwidth is less than or equal to 7.5 kHz.
 13. Thefrequency generating apparatus according to claim 1, wherein each ofsaid plurality of RF channels have equal bandwidths.
 14. The frequencygenerating apparatus according to claim 1, wherein said first and secondchannels comprise any two of said plurality of RF channels.
 15. Thefrequency generating apparatus according to claim 1, wherein said firstand second RF carriers occupy a single radio frequency channel atdifferent times.
 16. The frequency generating apparatus according toclaim 4, wherein said AFC circuit means tracks said SSB-AM reduced pilotcarrier when said radio receiver receives said second channel RFcarrier.
 17. The frequency generating apparatus according to claim 3,wherein said AFC circuit means tracks the approximate center frequencyof said narrowband FM data messages when said radio receiver receivessaid first channel RF carrier.
 18. The frequency generating apparatusaccording to claim 1, wherein said AFC circuit meansincludes:discriminator means for converting the frequency of said firstand second RF carriers to first and second tracking voltages; referencemeans for providing first and second reference voltages; and means forcomparing said tracking voltages with said reference voltages todetermine the frequency offset of said receiver injection signal. 19.The frequency generating apparatus according to claim 1, wherein saidAFC circuit means includes sweep means for sweeping said receiverinjection signal at a first rate for initial frequency tracking and forsweeping said receiver injection signal at a second rate for subsequentfrequency tracking.
 20. The frequency generating apparatus according toclaim 19, wherein said AFC circuit means includes logic means fordetermining said first and second sweeping rates.
 21. The frequencygenerating apparatus according to claim 1, wherein said AFC circuitmeans includes logic means for storing the present operational state ofsaid frequency generating apparatus, and for determining the nextoperational state of said frequency generating apparatus in response toa plurality of input signals.
 22. An automatic frequency control (AFC)apparatus for a radio receiver having a frequency generating means withAFC capability for generating a receiver injection signal, said receiverfurther having means for receiving voice and data signals over one of aplurality of radio frequency (RF) channels in response to said receiverinjection signal, said AFC apparatus comprising:circuit means responsiveto an AFC control signal for providing an AFC signal to said frequencygenerating means by tracking the frequency of one of either:(a) a firstRF carrier received form a first channel of said plurality of channels,said first channel having a predetermined bandwidth, said first RFcarrier having a given frequency band of voice signals modulated viasingle sideband (SSB) amplitude modulation (AM) within said firstchannel bandwidth, said SSB-AM further having a reduced pilot carrierwithin said first channel bandwidth; and of (b) a second RF carrierreceived from said first channel, said second RF carrier having highspeed data signals modulated via narrowband frequency modulation (FM)within said first channel bandwidth; and control logic means forproviding said AFC control signal to said AFC circuit means.
 23. Theautomatic frequency control apparatus according to claim 22, whereinsaid first channel bandwidth is less than or equal to 7.5 kHz.
 24. Theautomatic frequency control apparatus according to claim 22, whereineach of said plurality of RF channels have equal bandwidths.
 25. Theautomatic frequency control apparatus according to claim 22, whereinsaid first channel comprises any one of said plurality of radiofrequency channels.
 26. The automatic frequency control apparatusaccording to claim 22, wherein said AFC circuit means tracks said SSB-AMreduced pilot carrier when said radio receiver receives said first RFcarrier.
 27. The automatic frequency control apparatus according toclaim 22, wherein said AFC circuit means tracks the approximate centerfrequency of said narrowband FM data signals when said radio receiverreceives said second RF carrier.
 28. The automatic frequency controlapparatus according to claim 22, wherein said AFC circuit meansincludes:discriminator means for converting the frequency of said firstand second RF carriers to first and second tracking voltages; referencemeans for providing first and second reference voltages; and means forcomparing said tracking voltages with said reference voltages todetermine the frequency offset of said receiver injection signal. 29.The automatic frequency control apparatus according to claim 22, whereinsaid control logic means includes sweep means for sweeping said receiverinjection signal at a first rate for initial frequency tracking and forsweeping said receiver injection signal at a second rate for subsequentfrequency tracking.
 30. The automatic frequency control apparatusaccording to claim 22, wherein said control logic means includes meansfor storing the present operational state of said AFC circuit means, andfor determining the next operational state of said AFC circuit means inresponse to a plurality of input signals.
 31. A method of automaticallycontrolling the frequency of a radio transceiver having frequencygenerating means for providing a local oscillator signal with automaticfrequency control (AFC) capability and a transmitter injection signal,and having means for receiving and transmitting voice and data signalsover any one of a plurality of radio frequency (RF) channels, saidautomatic frequency controlling method comprising the steps of:(a)selecting a first channel of said plurality of channels by setting thefrequency of said frequency generating means such that said radiotransceiver receives said first channel; (b) sweeping the frequency ofsaid local oscillator signal within the channel bandwidth of said firstchannel; (c) detecting the presence of data signals angle modulated on afirst RF carrier on said first channel, and stopping said frequencysweeping upon such detection; (d) continuously adjusting the frequencyof said local oscillator signal to track the frequency of said first RFcarrier; (e) decoding said data signals to obtain transceiver channelassignment information; (f) selecting a second channel of said pluralityof channels in response to said channel assignment information bysetting the frequency of said frequency generating means such that saidradio transceiver receives said second channel; (g) continuouslyadjusting the frequency of said local oscillator signal to track thefrequency of a second RF carrier on said second channel; and (h)receiving voice signals amplitude modulated on said second channel. 32.The frequency controlling method according to claim 31, wherein saidamplitude modulation is single sideband (SSB) amplitude modulation (AM).33. The frequency controlling method according to claim 31, wherein saidangle modulation is narrowband frequency modulation (FM).
 34. Thefrequency controlling method according to claim 32, wherein said SSB-AMincludes a reduced pilot carrier.
 35. The frequency controlling methodaccording to claim 34, wherein the amplitude of said pilot carrier isapproximately 16 dB below peak envelope power.
 36. The frequencycontrolling method according to claim 34, wherein the peak amplitude ofsaid voice signals is approximately 3.0 dB below peak envelope power.37. The frequency controlling method according to claim 31, wherein saidangle modulation is transmitted without a pilot carrier.
 38. Thefrequency controlling method according to claim 31, wherein said anglemodulation is transmitted at full peak envelope power.
 39. The frequencycontrolling method according to claim 31, wherein said data signals arecentered at approximately 1.9 kHz above the lower edge frequency of saidfirst channel.
 40. The frequency controlling method according to claim31, wherein said data signals are digital data modulation atapproximately 2400 bps.
 41. The frequency controlling method accordingto claim 31, wherein the frequency deviation of said data modulation isapproximately 800 Hz.
 42. The frequency controlling method according toclaim 31, wherein said first channel bandwidth is less than or equal to7.5 kHz.
 43. The frequency controlling method according to claim 31,wherein each of said plurality of RF channels have equal bandwidths. 44.The frequency controlling method according to claim 31, wherein saidfirst channel comprises any one of said plurality of radio frequencychannels.
 45. The frequency controlling method according to claim 34,wherein said second RF carrier is said SSB-AM reduced pilot carrier. 46.The frequency controlling method according to claim 31, furtherincluding the steps of:storing the present operational state of saidradio transceiver; and determining the next operational state of saidradio transceiver in response to a plurality of input signals.